NCIS returned this month and the second episode of the new season was the show's 400th overall. A surprise guest also rejoined the series for the new episode, which aired on Tuesday this week: Mark Harmon's son, Sean Harmon, who slipped back into a younger version of his father's role, "Leroy Gibbs."

The 400th episode of the hit crime series explored how "Gibbs" came to the NCIS team and met medical examiner "Donald 'Ducky' Mallard" (David McCallum). But the flashback featured Harmon's eldest son returning to NCIS, after playing the young "Gibbs" six other times between 2008 and 2012.

NCIS: Mark Harmon is proud of his son Sean

In an interview with ET, Mark Harmon spoke about his son's performance and what it was like watching him in the role for episode 400. As a father, Harmon said, "there's pride involved" in collaborating with his son on the show—a role he earned, fairly, Harmon explained.

"I always think back to the first time they talked about doing a young 'Gibbs' thing here, and Sean was just out of school at that point. A director named Tony Wharmby said, 'Can he come in and read?'" Harmon recalled. "On his own he did and I'm proud of him taking his work seriously and how he approaches being an actor," the longtime NCIS star added.

Sean Harmon and Pam Dawber

NCIS season 18 aired show's 400th episode this week

In an interview with TV Guide, NCIS producer Frank Cardea had earlier revealed that the throwback episode was conceived to "write a love letter to the longtime fans." Mission accomplished, as over 10 million viewers tuned into season 18, episode 2—a.k.a. the 400th anniversary episode.

NCIS is off next week, but episode 3 will air on Dec. 8 on CBS. Worldwide viewers still await a premiere date for the new season. Until then, see how well you know Mark Harmon in our quiz here.

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Which of the following won the 1984 Grammy for Record of the Year?